As it turns out it's necessary to load the class Ext.form.field.File and not Ext.form.field.FileButton, even though the css class is used on the FileButton, not the FileField.Seems strange when FileField is not used.

Also Cmd is very buggy and flaky when using the web server with multiple projects on the same machine at the same time. Generating a new app sets some global namespace variables that cannot be reversed by refresh, build or restart the machine.
I had to recreate the entire project to reset that namespace back to the main project. The error is e.g.
"The following classes are not declared even if their files have been loaded: "testproject.Application". Please check the source code of their corresponding files for possible typos: "app/Application".
But there is no reference in the project itself to "testproject".